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1. Field of the Invention
The present invention is related to the field of electronics. In particular, the present invention is related to a method and apparatus for shifting the frequency spectrum of noise signals in electronic circuits.
2. Description of the Related Art
The existence of noise in electronic circuits may cause the circuits to malfunction. This is especially true in electronic communication devices wherein the existence of noise has the potential to drown out or distort the desired signals. The effects of noise in electronic circuits is illustrated with reference to FIGS. 1-3. FIG. 1 illustrates a block diagram of a conventional communication system 10. Communication system 10 includes a transmitter 12, a receiver 14 and a communication channel 16. Communication channel 16 may be either a wired or wireless communication channel.
FIG. 2 illustrates a sequence of waveforms, for example, as may be transmitted over communication system 10. Input signal 20 is a binary signal and comprises a sequence of binary values received by transmitter 12 of FIG. 1. Transmitter 12 converts the binary input signal into a bipolar signal 22, having a positive and a negative component. The bipolar signal 22 is transmitted to receiver 14 via communication channel 16. In one embodiment, transmitter 12 may modulate bipolar signal 22 onto a carrier wave prior to transmitting the signal to receiver 14.
As bipolar signal 22 propagates through communication channel 16, the signal may become deformed and attenuated as illustrated by waveform 24 in FIG. 2. Receiver 14 receives attenuated signal 24 and transmits it to equalizer (wave shaper) 18. Equalizer 18 amplifies and processes signal 24, producing a reconstructed signal 26, which is more or less a replica of bipolar signal 22.
At the input of equalizer 18 both the desired input signal 20, and unwanted noise signals are present and, hence, amplified by the equalizer. If the noise signals are of significant amplitude, i.e., if the noise signals have an amplitude larger than the desired signal and have a substantially similar frequency as the desired input signal 20, the noise signals may distort the desired signal. In this case, the equalizer would be unable to reconstruct the desired signal.
In order to propagate a desired input signal 20 through e.g., a communication system 10, switching circuits driven by clock signals often are employed in the receiver 14 and equalizer 18. As such, the undesired signals, for example, noise signals generated by the clock signals and the associated switching circuitry are at frequencies that are multiples of the clock signal frequency. The noise signal frequencies normally fall within the bandwidth of amplifiers in the communication system 10, and subsequently both the desired input signal 20 and the noise signals are amplified. Moreover, since the amplitude of the noise signals, of a particular frequency, generated by different switching circuits in the system are additive, noise signals from the various switching circuits may be superimposed on each other to form noise signals of greater amplitude, exaggerating the problem.
FIG. 3 illustrates a waveform showing the gain versus frequency curve of various signals at the input of equalizer 18. As FIG. 3 illustrates, the desired input signal 20 has a frequency of 100 MHz, a clock signal has a frequency of 25 MHz, harmonics of the clock signal have frequencies of 50 MHz, 100 MHz, 150 MHz etc. All the frequencies illustrated are present at the input of the equalizer 18. Hence, as the desired 100 MHz input signal 20 is amplified by the equalizer, so too are the undesired noise signals.
Conventional noise reduction methods and systems attempt to increase output signal quality by reducing the induced noise signals via the use of filter circuits. Using filter circuits to eliminate noise signals is not very effective, as noise signals induced by clock transitions, and with frequencies substantially similar to the desired signal frequency cannot be easily filtered out. What is needed, therefore, is a method and apparatus to reduce noise signals in electronic devices by shifting the frequency spectrum of noise signals to frequencies outside the bandwidth of the amplifiers, or to frequencies that are outside the operating frequency of the electronic system.